The present invention is directed to a method of operating an arc discharge lamp and to an arc discharge lamp in which lamp ignition is improved.
A problem with arc discharge lamps, particularly arc discharge lamps that are mercury-free, is that relatively low, reproducible and consistent breakdown voltages have been difficult to achieve. The breakdown voltage is the voltage at which lamp ignition begins, and erratic breakdown voltages can hinder or completely inhibit lamp ignition.
In such lamps, the interaction of the chemical fill with the arc tube components releases iodine in vapor form over time. The released iodine condenses when the lamp is turned off. During lamp cool-down, the iodine condenses randomly within the arc tube over a period of time that may exceed eight minutes, depending on the thermal inertia of the arc tube. The iodine is not static during this period and migrates within the arc tube to condense on surfaces that are only slightly cooler than ambient. The electrode tips and shanks (typically tungsten) are particularly good areas for iodine condensation because they are connected to cooler locations outside the lamp and are thermally conductive.
The growth of small dendritic crystals of iodine on the electrode tips is a particular problem for lamp ignition. During starting, the dendrites vaporize and reduce the E/N ratio locally (the EIN ratio is the local electric field strength per atomic density). The vaporizing dendrites loose their sharp surfaces to reduce E locally and increase atomic density to increase N locally. The net result of decreasing the E/N ratio is to quench the discharge and inhibit successful ignition.
The ignition properties of a lamp may be characterized by measuring its DC breakdown voltage. DC breakdown voltage represents the lowest voltage that can cause a sustained breakdown discharge to form.
FIG. 1 depicts a sequence of DC breakdown voltage measurements for a lamp not incorporating the present invention. Note that breakdown voltages were initially erratic and remained inconsistent even after 50 breakdown discharges. The initial breakdown voltages were on the order of 11-12 kv, and subsequent breakdown voltages decreased until a plateau was reached at about the 36th breakdown, although some higher voltages occasionally occurred. Small dendrites of iodine initially were observed on the surfaces of the electrodes, particularly the electrode tips, and with each breakdown discharge some of the dendrites were vaporized. Eventually enough breakdown discharges took place so that little or no iodine was left on the surfaces of the electrodes and DC breakdown voltages returned to more normal values.